Rails - Oct 2006 - Ruby For Rails Bind and Unbind question

When I run the following example:

class A 
  def a_method 
    puts "Definition in class A" 
  end 
end 
class B < A 
  def a_method 
    puts "Definition in class B (subclass of A)" 
  end 
end 
class C 
  def call_original 
    A.instance_method(:a_method).bind(self).call 
  end 
end
c = C.new
c.call_original

I get the following error:

TypeError: bind argument must be an instance of A

method bind	in c.rb at line 13
method call_original	in c.rb at line 13
at top level	in c.rb at line 17

Can someone please tell me why it is failing? TIA.

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Hi --

On Tue, 10 Oct 2006, Bala Paranj wrote:
>
> When I run the following example:
>
> class A
>  def a_method
>    puts "Definition in class A"
>  end
> end
> class B < A
>  def a_method
>    puts "Definition in class B (subclass of A)"
>  end
> end
> class C
>  def call_original
>    A.instance_method(:a_method).bind(self).call
>  end
> end
> c = C.new
> c.call_original
>
> I get the following error:
>
> TypeError: bind argument must be an instance of A
You''ve combined two related examples, but you''ve left out an
important
bit:

   class C < B
   end

(on page 358)  The class C declaration on p. 359 is a reopening of C,
so it doesn''t have the < B part.  If you want to do it all together,
just change "class C" to "class C < B".


David


-- 
                   David A. Black |
dblack-TKXtfPMJ4Ozk1uMJSBkQmQ@public.gmane.org
Author of "Ruby for Rails"   [1] | Ruby/Rails training &
consultancy [3]
DABlog (DAB''s Weblog)        [2] | Co-director, Ruby Central, Inc.  
[4]
[1] http://www.manning.com/black | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com    | [4] http://www.rubycentral.org

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Thank you. In the book example on const_missing, why does the output print 1
twice?

class C 
  def self.const_missing(const) 
    puts "#{const} is undefined—setting it to 1." 
    const_set(const,1) 
  end 
end 
puts C::A 
puts C::A 


 
> You''ve combined two related examples, but you''ve left out
an important
> bit:
> 
>    class C < B
>    end

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Why wouldn''t it?

The first puts statement calls const_missing() which returns 1 via
const_set().  puts takes it and prints it.  The second time puts is
called, C::A is defined to 1, so the statement functions as you would
expect it.


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Hi --

On Tue, 10 Oct 2006, Bala Paranj wrote:
>
> Thank you. In the book example on const_missing, why does the output print
1 twice?
>
> class C
>  def self.const_missing(const)
>    puts "#{const} is undefinedsetting it to 1."
>    const_set(const,1)
>  end
> end
> puts C::A
> puts C::A
It''s to show you that the first time, it calls const_missing and gives
you the longer message, but the second time, the constant is no longer
missing so it just prints 1.


David

-- 
                   David A. Black |
dblack-TKXtfPMJ4Ozk1uMJSBkQmQ@public.gmane.org
Author of "Ruby for Rails"   [1] | Ruby/Rails training &
consultancy [3]
DABlog (DAB''s Weblog)        [2] | Co-director, Ruby Central, Inc.  
[4]
[1] http://www.manning.com/black | [3] http://www.rubypowerandlight.com
[2] http://dablog.rubypal.com    | [4] http://www.rubycentral.org

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